SecurityPkg/DxeImageVerificationLib: assign WinCertificate after size check

Currently the (SecDataDirLeft <= sizeof (WIN_CERTIFICATE)) check only
guards the de-referencing of the "WinCertificate" pointer. It does not
guard the calculation of the pointer itself:

  WinCertificate = (WIN_CERTIFICATE *) (mImageBase + OffSet);

This is wrong; if we don't know for sure that we have enough room for a
WIN_CERTIFICATE, then even creating such a pointer, not just
de-referencing it, may invoke undefined behavior.

Move the pointer calculation after the size check.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Wenyi Xie <xiewenyi2@huawei.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2215


Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200901091221.20948-3-lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Wenyi Xie <xiewenyi2@huawei.com>
Reviewed-by: Min M Xu <min.m.xu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>